BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for grouping unit cells to make a battery pack of good performance and, more particularly, to a method for grouping unit cells of a similar impedance spectrum using pattern matching technology to optimize the performance of a battery pack made of primary or secondary batteries as the unit cells connected in series, in parallel or in combination.
2. Related Prior Art
As well known in the art, it is desirable for the good performance of a battery pack to select unit cells of a similar characteristic in the manufacture of the battery pack with unit cells connected in series, in parallel, or in combination of them. But the general method of evaluating a battery pack based on the initial capacity is unsatisfactory for securing the similarity for the unit cells of the battery pack, because the unit cells of the same capacity may be different from one another in aging speed and internal characteristic, such as impedance spectrum.
In the manufacture of a battery pack with a plurality of unit cells connected in series, in parallel or in combination of them, the battery pack can have an enhanced performance when the unit cells have a most similar impedance spectrum representing the internal characteristic. Contrarily, the battery pack, if built with unit cells of a different impedance spectrum, may have a deterioration of capacity or a high aging speed due to the difference in internal characteristic among the unit cells.
Until now, the measurement of impedance has been adapted to sort out defective unit cells. For example, the measurement of impedance at a fixed frequency for a short time of less than one second, as disclosed in U.S. Pat. No. 3,873,911, can be employed for detection of unit cells having a lower capacity than the average value. But the measurement of impedance at a fixed high frequency cannot always predict the accurate capacity of the unit cells. This is because the individual unit cells have a different tendency to increase the impedance in the low frequency region, which means that the unit cell of high impedance at 1 kHz may exhibit a low impedance value in the low frequency region.
The method of selecting unit cells to make a battery pack based on the measurement of open circuit voltage may also present erroneous results, because the open circuit voltage does not represent the accurate internal characteristic of the unit cells and varies insensitively according to the discharge state of the unit cells.
For quality control of the batteries to sort out defective batteries, there have been many methods proposed for analyzing the impedance spectrum of batteries. These methods are based on the measurement of dynamic resistance at a fixed frequency (U.S. Pat. No. 3,873,911); variations of voltage and capacity during a discharge (European Patent No. 0119547); response signals fed back from a battery upon applying a charging signal (U.S. Pat. No. 3,808,487); difference of phase angle between AC voltage across the battery and AC current applied to the battery (U.S. Pat. No. 3,984,762); the measurement of internal resistance of lithium-iodine batteries (U.S. Pat. No. 4,259,639); difference of argument between impedance values measured at two different frequencies (U.S. Pat. No. 4,743,855); the measurement of angle built by straight line between impedance points at several frequencies in Real vs. Imaginary part plot (Nyquist plot) (U.S. Pat. No. 5,241,275); and the analysis of parameters obtained by fitting impedance spectrum to an equivalent circuit model (U.S. Pat. No. 6,208,147).
These conventional methods have the same problem in that only a part of information regarding the internal characteristics of the battery obtained from the measurement can be used for the selection of batteries in such a simple way as to determine the quality of the battery as “good” or “defective”. As disclosed in U.S. Pat. No., 6,118,275, for example, the voltage response to current pulse measured for one second with 1 millisecond sampling rate has information about impedance at five different frequencies. Impedance of each frequency has its own unique information about the system. However, only one frequency is used for correlation in the prior art, and the other information is lost.
As described above, the method for analyzing the internal characteristic of batteries by fitting the impedance spectrum to a physically appropriate equivalent circuit model or separation of the impedance measurements into a capacitance component and a resistance component involves complicated procedures.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for grouping unit cells of a similar impedance spectrum using the pattern matching technology without any complex procedures; fitting the impedance spectra to a physically appropriate equivalent circuit model or separating the impedance measurements into a capacitance component and a resistance component in the analysis of impedance spectrum.
It is another object of the present invention to provide a method for grouping unit cells of a similar impedance spectrum using the pattern matching technology so as to optimize the performance of a battery pack made of the unit cells connected in series, in parallel, or in combination of them.
To achieve the objects of the present invention, there is provided method for grouping unit cells of a similar impedance spectrum to make an optimal battery pack, the method including: (a) measuring the impedance spectrum of the individual unit cell in a predetermined frequency region at the same temperature and in the same state of charge; (b) digitizing the difference of impedance spectrum among the unit cells into relative value using the pattern matching technology; and (c) selecting two or more unit cells having the smallest difference in the digitalized impedance spectrum.
The present invention is to analyze all the frequency regions of the measured impedance spectrum and to select the unit cells to make an optimal battery pack.
Also, the present invention is to select unit cells of a most similar internal characteristic using the pattern matching technology without a complicated process of fitting the impedance spectra to a physically appropriate equivalent circuit model or separating the impedance measurements into a capacitance component and a resistance component in the analysis of the impedance spectrums.
While other methods of selecting unit cells include analyzing a statistically significant number of unit cells before selecting unit cells and sorting unit cells according to this analysis result, the present invention only compares impedance spectra of the unit cells without a need of the analytical process and can be immediately applied to any kind of unit cells without a preliminary analysis.
The present invention is analyzing the impedance spectrum of electrochemical power sources, of which the measurement is an indirect and non-destructive way of determining the internal characteristic of the power sources, using the pattern matching technology and selecting the power sources of a most similar impedance spectrum, i.e., a most similar internal characteristic to make a battery pack. Thus, the present invention is more efficient than the real-time discharge method, which is a destructive test taking a long time in measurement, and more useful for grouping unit cells with good reliability and guaranteeing the performance of the battery pack, compared with the correlation method using parameters.
(d) The groups of batteries having a smallest standard deviation of the impedance spectra are selected to make battery packs, in step 106. To demonstrate the efficiency of battery pack with batteries of a similar impedance spectrum, two batteries having the smallest standard deviation of impedance spectrum, two batteries with the largest standard deviation, and two batteries with in-between standard deviation are selected to make serial and parallel battery packs. Then, it is checked in step 107 if the number of the remaining batteries is less than the number N of the batteries used to make one battery pack as entered in step 102. If the number of the remaining batteries is greater than N, the procedure returns to step 104 to exam the impedance spectra to make the battery packs. Otherwise, the process of making battery packs ends. The three groups of battery packs are marked with “Minimum”, “Maximum”, and “Middle”, respectively. The charge/discharge results of the serial and parallel battery packs sorted in three groups are presented in FIGS. 6 and 7, which show the battery packs of the Minimum group are superior in initial capacity and performance characteristic for continuous charge/discharge to those of the Maximum and Middle groups. The battery packs of the Maximum and Middle groups have similar performances, such as capacity or aging speed even though they differ from each other in the internal characteristic. This reveals that the performance of the battery pack is largely dependent upon the difference of the internal characteristic between the unit cells in the battery packs. Accordingly, it is most preferable to make a battery pack with unit cells having most similar internal characteristics, i.e., the smallest relative difference in impedance spectrum and thereby to reduce deterioration of the performance of the battery packs.